Source: BIO 311C, Intro to Biology, University of Texas at Austin
Difficulty: Introductory | Prerequisites: Basic understanding of cell structure, plasma membrane, and proteins.
Cell signalling is how cells talk to each other and coordinate their behaviour, and it sits at the heart of virtually every process in multicellular life: development, immunity, nervous system function, hormone regulation. This topic builds on your knowledge of membrane structure and protein function. If you are comfortable with the idea that proteins change shape when they bind ligands, you are ready. The concepts here feed directly into later units on gene regulation, the cell cycle, and cancer biology.
Cells communicate by releasing signalling molecules (ligands) that bind to specific receptors on or inside target cells. The signal is relayed through a chain of molecular events (transduction) that ends in a cellular response such as gene expression or enzyme activation. Signalling can be local (paracrine, autocrine, synaptic) or long-distance (endocrine), and even bacteria do it through quorum sensing.
Cell signalling
The process by which cells communicate with one another through chemical or physical signals to coordinate responses.
In simple terms, this is how one cell tells another cell what to do.
Signal transduction
The conversion of an extracellular signal into an intracellular response, carried out through a series of molecular relay steps.
Think of it as a chain of dominoes inside the cell: the first one tips when the signal arrives at the membrane, and the last one triggers the response.
Ligand
A molecule that binds specifically to a receptor protein, triggering a change in that receptor's shape or activity.
In simple terms, the ligand is the "message" and the receptor is the "mailbox."
Receptor
A protein (usually on or in a cell) that recognises and binds a specific signalling molecule, initiating a cellular response.
Quorum sensing
A form of bacterial communication in which cells release and detect signalling molecules to gauge local population density and coordinate group behaviours.
Think of it as bacteria taking a headcount before deciding to act.
Paracrine signalling
Short-range signalling in which a cell secretes a chemical that targets nearby cells.
Autocrine signalling
Signalling in which a cell releases a molecule that binds to receptors on its own surface, effectively signalling to itself.
Endocrine signalling (hormonal signalling)
Long-distance signalling in which hormones are released into the bloodstream and travel to distant target cells.
Synaptic signalling
Rapid, short-range signalling in the nervous system where a neurotransmitter is released across a synapse to a target cell.
Second messenger
A small, non-protein molecule (such as cAMP or Ca²⁺) that relays a signal inside the cell after a receptor on the cell surface has been activated.
Think of it as the internal courier that carries the message from the front desk deeper into the building.
Phosphorylation cascade
A chain of reactions in which one protein kinase activates the next by adding a phosphate group, amplifying the original signal at each step.
Cell signalling is the umbrella term for communication between cells and tissues. It covers the coordination of responses within an organism, including induction, where one cell's signal determines the developmental path of another.
Signal transduction is the specific process by which an extracellular signal is transmitted via a pathway across the membrane, initiating an intracellular change or response.
Both prokaryotic and eukaryotic cells regulate gene expression in response to environmental conditions. Eukaryotic cells go further: gene expression also regulates development and gives rise to the different cell types that make multicellularity possible.
Two sources of developmental information shape the early embryo:
Cytoplasmic determinants (molecules already present in the egg)
Induction by nearby cells (signals from neighbours)
The plasma membrane is selectively permeable. Small hydrophobic molecules (O₂, CO₂, steroids) cross freely. Small uncharged polar molecules (water, ethanol) can cross slowly. Large polar molecules (glucose) and ions (Na⁺, K⁺, Cl⁻, Ca²⁺) cannot cross without transport proteins.
Cell signalling likely evolved first in prokaryotes and single-celled eukaryotes, and was later adopted by multicellular organisms.
Bacteria use the concentration of signalling molecules to sense local population density, a process called quorum sensing. Once the chemical concentration crosses a threshold, coordinated behaviours kick in: antibiotic production, bioluminescence, conjugation, and virulence factor expression.
Biofilm formation is a related process. Individual rod-shaped cells aggregate into pockets of density, form spore-forming structures (fruiting bodies), and change their behaviour as the community grows.
Local signalling covers communication between cells that are near each other. There are several mechanisms:
Cell-to-cell contact (cell-cell recognition): surface molecules on one cell fit receptors on an adjacent cell directly. No secreted chemical is needed.
Cell junctions: animal cells use gap junctions and plant cells use plasmodesmata to connect the cytoplasm of adjacent cells directly. Signalling substances in the cytosol can pass freely through these connections.
Paracrine signalling: a cell secretes a signalling molecule that diffuses to and acts on nearby target cells.
Autocrine signalling: a cell secretes a molecule that binds to receptors on its own surface.
Plants and animals use hormones for long-distance communication. In endocrine signalling, specialised cells release hormones into the circulatory system, which carries them to distant target cells.
The target cell must have the right receptor for the hormone. Estrogen, for instance, binds to specific receptors inside many kinds of cells, each with different downstream effects, which is why a single hormone can mediate such a wide range of responses across the body.
Synaptic signalling is a specialised form of local signalling in the animal nervous system. It occurs when a neurotransmitter is released from a presynaptic neuron in response to an electrical signal (action potential) and diffuses across the synaptic cleft to the postsynaptic cell.
Key features of neurons:
Highly specialised in shape and membrane protein composition
Maintain ion concentration gradients across their membrane: higher Na⁺ and Cl⁻ outside, higher K⁺ inside
The inside of the cell is more negative at rest (resting membrane potential)
Signals travel along the axon as action potentials, involving the opening and closing of voltage-gated sodium and potassium channels
The sequence at a synapse runs: action potential arrives at the axon terminal, voltage-gated Ca²⁺ channels open, Ca²⁺ influx triggers neurotransmitter release from synaptic vesicles, neurotransmitter crosses the synaptic cleft, binds receptors on the postsynaptic membrane, and produces a response in the target cell. Reuptake molecules and enzymes then clear the neurotransmitter from the cleft.
Students often think that only multicellular organisms use cell signalling. They do not: prokaryotes use quorum sensing, one of the earliest evolved forms of signalling.
Students sometimes confuse paracrine and endocrine signalling. Paracrine is local (short-range diffusion to nearby cells). Endocrine is long-distance (hormones travel through the bloodstream).
A common error is thinking that a non-target cell "rejects" a signal. It does not. A non-target cell simply lacks the receptor for that ligand, so nothing happens.
Students often assume synaptic signalling is long-distance because neurons are long. The signal itself (neurotransmitter release across the synaptic cleft) is a local event. The long-distance component is the electrical signal travelling along the axon, not the chemical signal at the synapse.
⚠️ Know the three types of local signalling (paracrine, autocrine, cell junction) and be able to distinguish them from endocrine and synaptic signalling. Exam questions frequently ask you to classify a scenario.
⚠️ Be able to explain quorum sensing and give at least one example of a behaviour it coordinates (bioluminescence, biofilm formation, virulence factor expression).
⚠️ Understand why a target cell responds to a signal and a non-target cell does not (presence or absence of the correct receptor).
⚠️ Gap junctions vs. plasmodesmata is a classic compare-and-contrast question: gap junctions are animal, plasmodesmata are plant, both allow direct cytoplasmic connection.
⚠️ The ion distribution across a neuron's membrane (Na⁺ and Cl⁻ high outside, K⁺ high inside, interior negative at rest) is foundational for understanding action potentials.
True or False: Quorum sensing is a form of long-distance signalling. (False: it is local, based on concentration of signalling molecules in the immediate environment.)
Fill in the blank: In animal cells, ______ connect the cytoplasm of adjacent cells directly. (gap junctions)
True or False: A non-target cell actively degrades a signalling molecule it does not recognise. (False: it simply lacks the receptor, so no binding occurs.)
Fill in the blank: In endocrine signalling, hormones travel to distant target cells via the ______. (circulatory system / bloodstream)
True or False: The inside of a neuron is more positive than the outside at rest. (False: the inside is more negative at rest.)
Q: Name the three categories of local signalling in animal cells and briefly describe each.
A: (1) Direct contact (cell-cell recognition), where surface molecules on one cell bind receptors on an adjacent cell. (2) Cell junctions (gap junctions), where the cytoplasm of adjacent cells is directly connected, allowing signalling molecules to pass freely. (3) Paracrine signalling, where a cell secretes a chemical messenger that diffuses to nearby target cells.
Q: What is quorum sensing, and why is it significant for bacteria?
A: Quorum sensing is a process in which bacteria release and detect signalling molecules to assess local population density. When the concentration of these molecules crosses a threshold, it triggers coordinated group behaviours such as biofilm formation, bioluminescence, antibiotic production, and virulence factor expression.
Q: Explain why estrogen can produce different effects in different cell types.
A: Estrogen binds to intracellular receptors found in many cell types, but the downstream proteins and pathways differ from one cell type to another. The same hormone therefore triggers different gene expression patterns depending on which proteins the target cell has available.
Q: Compare and contrast gap junctions and plasmodesmata.
A: Both are cell junctions that directly connect the cytoplasm of adjacent cells, allowing small signalling molecules to pass between them. Gap junctions occur in animal cells. Plasmodesmata occur in plant cells and are small pores through the cell wall.
Q: Describe the sequence of events at a chemical synapse, from action potential arrival to signal termination.
A: An action potential reaches the axon terminal. Voltage-gated Ca²⁺ channels open, and calcium flows in. The Ca²⁺ influx causes synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft. The neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic cell, triggering a response. Enzymes in the cleft and reuptake mechanisms then clear the neurotransmitter, terminating the signal.
This material connects directly to membrane transport (Chapter on membranes): understanding which molecules can and cannot cross the lipid bilayer explains why some signals need surface receptors and others can enter the cell directly. It also sets the stage for gene regulation, since many signalling pathways end by activating transcription factors that switch genes on or off. Later units on cancer biology revisit signalling extensively, because cancer often arises from mutations in signalling proteins (receptors, kinases, or growth factors) that cause uncontrolled cell division.
Cell signalling, cell communication, signal transduction, ligand, receptor, quorum sensing, biofilm, paracrine, autocrine, endocrine, hormonal signalling, synaptic signalling, neurotransmitter, gap junctions, plasmodesmata, cell junctions, action potential, ion gradient, resting membrane potential, induction, cytoplasmic determinants, selective permeability, BIO 311C, intro biology, UT Austin